<template>
	<ul class="list">
		<li class="item" v-for="item of letters" :key="item" :ref="item" @touchstart="handleTouchStart" @touchmove="handleTouchMove"
		 @tocuhend="handleTouchEnd" @click="handleletterClick">
			{{item}}</li>
	</ul>
</template>

<script>
	export default {
		name: 'CityAlphabet',
		props: {
			cities: Object
		},
		data() {
			return {
				touchStatus: false,
				startY: 0,
				timer: null
			}
		},
		updated() {
			this.startY = this.$refs['A'][0].offsetTop
		},
		computed: {
			letters() {
				const letters = []
				for (let i in this.cities) {
					letters.push(i)
				}
				return letters
			}
		},
		methods: {
			handleletterClick: function(e) {
				//console.log(e.target.innerText)
				this.$emit('change', e.target.innerText)
			},

			handleTouchStart() {
				this.touchStatus = true
			},

			handleTouchMove(e) {
				if (this.touchStatus) {
					if (this.timer) {
						clearTimeout(this.timer)
					}
					// 使用截流降低函数调用的频率
					this.timer = setTimeout(() => {
						// const startY = this.$refs['A'][0].offsetTop
						const touchY = e.touches[0].clientY - 79
						const index = Math.floor((touchY - this.startY) / 20)
						if (index >= 0 && index < this.letters.length) {
							this.$emit('change', this.letters[index])
						}
					}, 16)

				}
			},

			handleTouchEnd() {
				this.touchStatus = false
			}
		}
	}
</script>

<style scoped="scoped">
	.list {
		display: flex;
		flex-direction: column;
		justify-content: center;
		position: absolute;
		top: 1.58rem;
		right: 0;
		bottom: 0;
		width: .4rem;
	}

	.item {
		line-height: .4rem;
		text-align: center;
		color: #00BCD4;
	}
</style>
